When measuring length with the aid of interferometers, the wavelength of the laser radiation utilized in air is used as the measuring standard or material measure. Hereinafter, this is denoted as light wavelength. The light wavelength is dependent on specific ambient parameters like, for instance, the temperature, the pressure, the humidity and the precise gas composition. Therefore, knowledge of the correct light wavelength or compensation of environmental influences during the measurement is necessary for a correct measurement of length.
Laser interferometers, whose measuring path and/or reference path take(s) a course in air, are therefore subject to considerable measurement fluctuations which are caused by local fluctuations in the refractive index of the air. The stability, reproducibility and precision of such laser interferometers is thereby restricted to 1·10−6 (workshop conditions) up to 1·10−7 (good laboratory conditions) relative to the measuring path. Since the strongest fluctuations lie in the 0-10 Hz frequency range, they also adversely affect measurements which are performed in a relatively brief time. Therefore, today's demands on the stability and reproducibility of linear measurements in the electronics and semiconductor industry can no longer be satisfied. Typically, they lie in the range of 1·10−8 to 1·10−9 (i.e., 0.2 nm to 2 nm, given an average measuring distance of 20 cm) over time intervals of a few minutes.
A number of design approaches to detect and compensate for such fluctuations of refractive index in air have already been described.
In a first variant, the different ambient parameters such as air temperature, air pressure and air moisture are detected with the aid of suitable sensors, and a corrected or effective light wavelength is determined using what is called the Edlen formula. For instance, such a method is described in the dissertation by Jens Flügge, “Vergleichende Untersuchungen zur messtechnischen Leistungsfähigkeit von Laserinterferometern und inkrementellen Maβstabmesssystemen” (“Comparative Studies with Respect to the Metrological Performance of Laser Interferometers and Incremental Scale Measuring Systems”), RWTH Aachen (D82), February 1996, ISBN 3-89429-683-6, Pg. 13-14. The disadvantage in this procedure is that the various parameters with respect to the ambient conditions are determined only at discrete points and only in the vicinity of the optical measuring axis. The exact characteristic of these parameters along the measuring axis is therefore only approximately determined, resulting in turn in inaccuracies when determining the effective light wavelength, and therefore in the actual linear measurement. In addition, as a rule, the different sensors have significant response times, so that short-duration fluctuations of the parameters possibly occurring along the measuring axis are likewise not correctly measurable. Because of these shortcomings, the accuracy of this variant for sensing and compensating for fluctuations in the refractive index in the case of interferometric linear measurements in air must be regarded as limited.
A second variant provides for determining the air wavelength with the aid of a refractometer. Such a method is also described in the dissertation by Jens Flügge already mentioned above, “Vergleichende Untersuchungen zur messtechnischen Leistungsfähigkeit von Laserinterferometern und inkrementellen Maβstabmesssystemen” (“Comparative Studies with Respect to the Metrological Performance of Laser Interferometers and Incremental Scale Measuring Systems”), RWTH Aachen (D82), February 1996, ISBN 3-89429-683-6, Pg. 15-16. In principle, the same problems are apparent in this case as in the first variant; in addition, this procedure must be classified as relatively complicated and therefore costly.
U.S. Pat. No. 6,501,550 describes a third variant for correcting the light wavelength in interferometric measuring methods which provides an acousto-optical interferometer system for this purpose. With the aid of a sound transmitter and a sound receiver, the sound propagation time along the measuring axis is ascertained, and the sound velocity is determined by linking the measured sound propagation time to the position known via the optical interferometer measurement. Since in known manner, the sound velocity is in turn a function of the prevailing ambient conditions, an instantaneous average air temperature along the measuring axis may be determined via a suitable correction function. The air temperature determined in this manner is then used in turn as input variable for the method, already described at the outset, for determining the average light wavelength with the aid of the Edlen formula, etc. It must be cited as an advantage of this method that the sound velocity averaged along the measuring axis is used as a measured quantity going into the correction. Therefore, compensation may be made for any existing fluctuations in the temperature along the measuring axis, as well. However, the disadvantage in this case is that, because of the substantially greater wavelength of the sound wave (2-5 mm) and the stronger diffraction effects associated with it, the transverse expansion of the sound wave is 20-100 times greater than that of the light wave. This results in two crucial disadvantages of this method: First of all, sound reflections and deflections at adjacent machine parts, which can hardly be avoided, lead to interference signals which invalidate the measuring result of the ultrasonic measurement. Secondly, the ultrasonic wave covers a markedly greater volume of air around the measuring axis, so that air-temperature fluctuations outside of the light wave of the interferometer likewise result in measuring errors.
In a fourth variant, denoted hereinafter as multi-wavelength interferometry, the vacuum wavelength may be corrected by the use of a plurality of wavelengths. To this end, the dispersion behavior of air is used for the correction. With respect to methods of this kind, reference is made to U.S. Pat. No. 5,404,222 or to U.S. Pat. No. 5,764,362, for example, which describe corresponding procedures in detail. To be regarded as disadvantageous in the fourth variant of the environmentally compensated, interferometric measuring methods is the relatively large expenditure resulting for the necessary optical frequency multiplication of the highly accurate laser light source. Furthermore, a considerable measuring uncertainty in the case of the necessary determination of the dispersion behavior of air must always be taken into account, since the dispersion effect utilized is very small. Therefore, the measuring fluctuations are able to be reduced only slightly via this method.